Hot and corrosion-resistant steel

ABSTRACT

The invention relates to metallurgy, more specifically to chromium- and nickel alloyed steels which are used in reactors (retorts) for magnesium-thermic production of spongy titanium. The inventive hot- and corrosion-resistant steel comprises iron (Fe) in the form of a base, carbon (C), nitrogen (N), manganese (Mn), silicium (Si), chromium (Cr), nickel (Ni), vanadium (V), at least one type of rare-earth metal (P3M) from a group of cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium (Nd), said steel also contains titanium (Ti) and at least one element from a group of niobium (Nb), tantalum (Ta), zirconium (Zr) and hafnium (Hf) which are used in the form of carbide- and nitride solvents at the following component ratio, in mass %: C 0.04-0.15; N 0.01-0.25; Si 0.1-1.0; Mn 3.0-12.5; Cr 1.0-15.0; Ni 1.0-7.0; V 0.05-0.5; one or several elements of a group of Ce, La, Pr, Nd 0.0001-0.01; and Ti 0.1-2.0; and one or several elements of a group of Nb, Ta, Zr, Hf 0.05-0.2. Said invention makes it possible to increase the hot and corrosion-resistance in aggressive media used for the magnesium-theremic production of spongy titanium and to reduce the contamination thereof with detrimental impurities.

The invention relates to metallurgic industry, namely, to corrosion resistant steel doped with chromium, nickel and manganese which are used in reactors for magnesium-thermal production of spongy titanium.

There exists a problem of increasing durability of reactors (retorts) which are used in magnesium-thermal production of spongy titanium; currently their average durability is about 30 production cycles. Such a fair durability of the retorts is defined by severe operating conditions: their internal surface is subject to high temperatures (800°-1000° C.) of melts of manganese chloride (MgCl₂) and metallic manganese (Mg), liquid and vaporous titanium tetrachloride (TiCl₄), and lower titanium chlorides when their external surface is subject to high-temperature gaseous atmosphere (1000°-1020° C.) consisting of the air in reduction and separation furnaces. Moreover, the retorts 1500 mm in diameter are subject to mechanical loads ranging from 4 to 8 ton-force. In the process of production the retorts are rejected both in wear of the walls due to corrosion and in elongation of the retorts due to their plastic deformation.

Steel 12X18H10T is available which is used for retorts for magnesium-thermal production of spongy titanium/Putina O. A. et al. Influence of various factors on service life of retorts of equipments for magnesium-thermal production of titanium <<Tsvetnyje metally>>, 1979, #9, pp. 71-72/containing according to GOST 5632-72 (in mass percent) C≦0.12; Si≦0.8; Mn≦2.0; Cr 17.0-19.0; Ni 9.0-11.0; S≦0.020; P≦0.35; Ti 0.6-0.8.

According to the data available [1], during the service life of the retorts made of 12X18H10T steel, wear of the bottom was 4-8 mm, elongation 150-200 mm which fact demonstrates poor corrosion and heat resistance of the known steel in applications discussed above.

Also, 10X23H18 is available that was studied and tested commercially for its practicability for manufacturing retorts for equipment for magnesium-thermal titanium production/Putina O. A., Putin A. A., Improvement of air-tightness and reliability of equipments for magnesium-thermal production of spongy titanium. |Reports of the 1^(st) science and technical conference on titanium of CIS states. Moscow, 1994, pp. 176-189/. According to GOST 5632.73, this steel contains (in mass percent): C≦0.1; Si≦10; Mn≦2.0; Cr 22-25; Ni 17.0-200.

Increased chromium and nickel content known 10X23H18 steel allowed to increase durability of the retorts made of this steel up to 43 cycles with elongation of the retorts up to 82 mm; however, in this case contamination of the titanium sponge obtained in the retorts with nickel increased, though such contamination is strictly specified Ni≦0.04 % [3].

The most approximate in chemical composition and achieved technical results is corrosion resistant steel (utility patent of Ukraine #30921 A, IPC⁶ C22C 38/58, 1998), which besides iron contains (in mass percent): C 0.01-0.05; N 0.01-0.20; Mn 4.5-11.5; Cr 15.5-18.5; Ni 0.5-2.0; Cu 0.1-0.6; V 0.05-0.4; rare-earth metals (REM) 0.001-0.01.

Application of the corrosion resistant steel mentioned above for manufacturing retorts for equipment for magnesium-thermal titanium production allowed decreasing contamination of titanium sponge to 0.035% with nickel incoming from material of the retort at the cost of its dissolving by liquid manganese which is present in the retort. Together with decrease in contamination of titanium sponge with nickel resulted from its low concentration in the mentioned above corrosion resistant steel, some increase in its heat resistance was gained at the cost of alloying with nitrogen and manganese. However, the retorts made of the mentioned above steel chosen as a prototype did not possess adequate corrosion resistance against TiC1₄ and MgCl₂ due to heterogeneity of the structure [3].

As the basis for the invention a task was set to create a heat resistant (low-ductility at high temperatures) corrosion resistant steel possessing high resistance against aggressive media under conditions of magnesium-thermal titanium production and applicable both for manufacturing reactors (retorts) as a basic metal and for application as an internal protective layer, i.e. for manufacturing retorts of bimetals.

Goals of the task set are achieved by the fact that heat and corrosion resistant steel containing (besides iron as a basis) the following elements: nitrogen, manganese, silicon, chromium, nickel, vanadium, and at least one rare-earth metal (REM) from cerium, lanthanum, praseodymium, neodymium group contains additionally titanium and at least one element of niobium, tantalum, zirconium and hafnium group as carbide- and nitride formers with the following relationship of elements (in mass percent): carbon 0.04-0.15; nitrogen 0.01-0.25; silicon 0.1-1.0; manganese 3.0-12.5; chromium 1.0-15.0; nickel 1.0-7.0; vanadium 0.05-0.5; one or more REM of cerium, lanthanum, praseodymium, neodymium 0.0001-0.01 group; titanium 0.1-2.0; one or more elements from niobium, tantalum, zirconium and hafnium group 0.05-0.2.

Presence of vanadium in the steel by the invention provides binding of carbon and nitrogen atoms present in the steel into steady carbonitride phase which in large measure inhibits nitride formation of Cr₂N, CrN type and carbides of (FeCr)₂₃C₆ type; this considerably decreases depletion of the grain boundaries in chromium and increases resistivity of the steel against intercrystalline corrosion at high temperatures.

Additional introduction of titanium to heat and corrosion resistant steel by the invention and at least one of the elements of niobium, tantalum, zirconium and hafnium group allows together with vanadium already present to create a multicomponent modifying complex (vanadium-titanium—one or more elements of niobium, tantalum, zirconium and hafnium group) which imparted to the steel by the invention a mechanism for crushing the carbide and nitride phase. Functioning of this mechanism is due to the fact that in the liquid state at the stage of crystallization of the melt, titanium and niobium and such elements as tantalum, zirconium and hafnium acts as competitors in carbide and nitride formation. Combined application of such strong carbide and nitride formers allows obtaining carbide and nitride phase steady up to 1100-1150° C. with favorable rounded shape of grains which provides increase in creep limit of the steel by the invention at the specified temperature, and, therefore, decrease in the retorts deformability in the range of operating temperatures.

Moreover, additional introduction of titanium and one or more elements of niobium, tantalum, zirconium and hafnium group as carbide- and nitride formers increases corrosion stability of the steel by the invention in TiCl₄, and MgCl₂ medium which provides both prolongation of the service life of the retorts and decrease in contamination of spongy titanium produced in this process, with nickel, chromium and iron.

Relationship between the components of the heat and corrosion resistant steel by the invention is based on the following.

The upper value of the carbon content (0.15 mass percent) is the limit behind which mass release of embrittling high-chromium secondary phases begins that decreases elasticity of the steel by the invention. The lower value of the carbon content (0.04 mass percent) is limited by abrupt decrease in creep flow of the steel which may result in deformation of the retorts in the course of operation.

The upper value of the nitrogen content (0.25 mass percent) is due to its limiting solubility in chromium-nickel steels. The lower value of the nitrogen content (0.01 mass percent) is limited by sharp decrease in the strength limit and fluidity limit of the steel by the invention.

The upper value of the silicon content (1.0 mass percent) is the boundary up to which deoxidation of the steel by the invention is provided with its efficient heat resistance; above this limit a sharp decrease in its plasticity begins. The lower limit of the silicon content (0.1 mass percent) is limited by initiation of useful deoxidating effect of this element.

The upper value of the manganese content (12.5 mass percent) is limited by a sharp decrease in corrosion stability of the steel by the invention when manganese content exceeds the specified value. The lower value of the manganese content (3.0 mass percent) is limited by a probability of formation of ferrite or martensite which abruptly decreases the creep limit and corrosion stability of the steel by the invention.

The limiting values of the chromium content (1.0-15.0 mass percent) were selected from the conditions to provide the combination of sufficient heat resistance and corrosion stability of the steel by the invention under successive or simultaneous action of TiCl₄, MgCl₂ and liquid Mg, and also lower titanium chlorides. In these concentrations chromium in combination with titanium, niobium and vanadium stabilizes efficiently the austenite structure of the steel by the invention providing its heat resistance at the minimum permissible level.

The upper value of the nickel content (7.0 mass percent) is defined by beginning of its intensive solution in liquid magnesium in the case of contacting with it in the process of titanium reduction in the retort of the equipment for magnesium-thermal titanium production. The lower value of the nickel content (1.0 mass percent) is limiting by initiation of austenite-forming effect for obtaining a stable austenite structure of the steel by the invention. Only austenite structure can provide necessary characteristics of corrosion stability and high-temperature strength of the steel by the invention.

The upper value of the vanadium content (0.4 mass percent) is determined by initiation of its negative influence on high-temperature strength of the steel by the invention. The lower value of the vanadium content (0.05 mass percent) is determined by the amount of this element in the steel by the invention sufficient for initiation of forming independent carbides and nitrides of vanadium or multicomponent carbides of (V, Cr)₇C₃ type.

The limiting values of one or more rare-earth metals from cerium, lanthanum, praseodymium, neodymium group (0.001-0.01 mass percent) was chosen from considerations of their useful action in decreasing diffuse mobility of carbon and nitrogen atoms that hinders formation of such embrittling phases as carbides and nitrides at the grain boundaries, and also facilitate their crushing and uniform distribution in the structure of the claimed steel; due to this fact it becomes possible to decrease its brittleness.

The upper value of the titanium content (2.0 mass percent) is limited due to a possibility of over-alloying of the solid solution accompanied by decrease in plasticity of the steel by the invention. The lower value of the titanium content (0.1 mass percent) is due to initiation of its alloying and modifying efficiency which provides increase in strength characteristics of the steel by the invention, particularly in the range of operating temperatures.

Such elements as niobium, tantalum, zirconium and hafnium belong to the group of elements having a similar effect on the properties of the steel by the invention in the case of their introduction in its composition by one at a time or in combination. Therefore only their total content is of importance. The lower content of one or more elements of the specified group (0.05 mass percent) in the steel by the invention is chosen from conditions of initiation of their positive effect on high-temperature strength, and the upper value—abrupt decrease in their efficiency.

In the course of searching for the optimum composition of the steel by the invention a large number of laboratory meltings were carried out in the induction basic furnace of volume 1000 dm³, including the steels corresponding in composition to the parallel patents and prototype. The obtained castings were forged in billets 40×80×100 mm in size, which were hot-rolled to the thicknesses of 25, 20 and 16 mm; then the obtained test samples were quenched from 1080° C. into water and subjected to alkaline and acid etching for descaling. Corrosion resistance of the test samples was determined by gravimetric method after trials in melted magnesium at 700-800° C. with full dip of the samples.

In order to determine the creep limit comparative trials of test steels (samples 1-8) with standard steels 12X18H10T and 10X23H18 (samples 9

10, respectively) were performed (the later steels were chosen as the analogs) and corrosion resistant steel according to the utility patent of Ukraine #30921 A (sample #11), chosen as a prototype at 850° C. for 10,000 hours. The obtained experimental data are summarized in the Table 1.

As one can see from the Table 1, the test steel (sample #5) having the composition that corresponds to the steel by the invention, possesses the most favorable structure and the most optimum combination of mechanical and useful properties.

There exist two alternatives for practical use of the steel by the invention for manufacturing retorts for equipment for magnesium-thermal production of spongy titanium: of the steel by the invention or application of the steel by the invention as a bimetallic cladding layer. As a basis for bimetals for manufacturing the retorts the 12X18H10T and 10X23H18 and other steels may serve which were chosen as the analogs. In this case similarity of physical properties (especially thermal expansion coefficient) of the steels making up the bimetal is of the extreme importance. Data on physical properties of the specified bases of bimetals and cladding layer of the steel by the invention are given in the Table 2.

As one can see from the Table 2, physical properties (melting temperature, density, coefficient of elasticity and coefficient of thermal expansion) of 12X18H10T, 10X23H18 steels and the steel by the invention are essentially at the same level which proves a possibility to use the steel by the invention as a cladding bimetallic layer with bases of the mentioned above steels.

TABLE 1 Composition of the chemical element (in mass percent) Corrosion Rare- rate, Creep Sam- earth gram/ limit, ple C N Si Mn Cr Ni V Ti Nb metals Fe Structure cm²year Mpa 1 0.04 0.01 0.10 3.0 10.0 1.0 0.05 0.10 0.05 0.0001 rest M + F 2.4819 1.6 2 0.15 0.25 1.00 12.5 15.0 7.0 0.50 2.0 0.2 0.0010 rest A 2.3754 2.8 3 0.04 0.01 0.90 3.2 14.80 1.06 0.47 1.77 0.19 0.0010 rest F + 15% A 2.4791 2.2 4 0.14 0.21 0.15 12.3 10.43 6.56 0.08 0.14 0.08 0.0004 rest A 2.3366 3.5 5 0.12 0.04 0.80 8.2 13.10 3.25 0.24 0.81 0.10 0.0010 rest A 1.2703 3.6 6 0.10 0.025 0.80 9.8 14.0 6.0 0.36 0.85 0.15 0.0010 rest A 1.4163 3.6 7 0.08 0.024 0.71 10.4 13.9 4.1 0.31 0.78 0.11 0.0020 rest A 1.8214 3.2 8 0.06 0.019 0.40 4.2 12.8 2.8 0.14 1.4 0.08 0.0008 rest A + 10% F 2.1213 2.6 Analogues 9 ≦0.12 — ≦0.8 ≦2.0 17.0-19.3 9.0-11.0 — ≦0.6 — — rest A 3.1904 — 10 ≦0.10 — ≦0.8 13.0-15.0 13.0-15.0 2.8-4.5 — ≦0.6 — — rest A + 20% F 2.9704 1.2 Prototype 11 0.01-0.05 0.01-0.20 —  4.5-11.5 15.5-18.5 0.5-2.0 0.05-0.4 — Cu 0.001-0.01 rest A + 20% F 2.5040 1.4 0.1-0.6 A = austenite, M = martensite, F = ferrite

TABLE 2 Modulus of Coefficient of thermal Melting elasticity expansion, Density, temperature, E * 10⁻³ α * 10⁶ Steel grade Kg/m³ ° C. T, ° C. megapascal 20-100° C. 400-500° C. 600-700° C. 800-900° C. Bimetal base 12X18H10T 7920 1400-1425 20 203 15.5 19.7 20.8 20.6 10X23H18 7900 1400-1430 20 200 15.4 19.2 22.0 23.3 Cladding bimetallic layer Steel under 7800 1392-1429 20 198-202 16.0 19.2 20.3 21.0 invention

REFERENCES

-   1. Putina O. A., Putin A. A., Gulyakin A. I. Influence of various     factors on service life of retorts of equipments for     magnesium-thermal production of titanium. <<Tsvetnyje metally>>,     1979, #9; -   2. Putina O. A., Putin A. A. Improvement of air-tightness and     reliability of equipments for magnesium-thermal production of spongy     titanium. Reports of the 1st science and technical conference on     titanium of CIS states. Moscow, 1994; -   3. Mishchenko V. G., Tverdokhleb S. V., Omelchenko O. S. Fracture     growth of reduction equipments and admixtures in spongy titanium.     Vestnik dvigatelestrojenija. Zaporozhje: <<Motor Sich>> JSC, 2004,     pp. 135-137. 

1. Heat and corrosion resistant steel which besides iron (Fe) forming the basis, contains carbon (C), nitrogen (N), manganese (Mn), silicon (Si), chromium (Cr), nickel (Ni), vanadium (V), and at least one of the rare-earth metals of the cerium (Ce), lanthanum (La), praseodymium (Pr), neodymium (Nd) group, characterized in that it additionally contains titanium (Ti) and at least one element of the niobium (Nb), tantalum (Ta), zirconium (Zr), hafnium (Hf) group as carbide and nitride formers with the following relationship of the elements (in mass percent): C 0.04-0.15; N 0.01-0.25; Si 0.1-1.0; Mn 3.0-12.5; Cr 11.7-15.0; Ni 1.0-3.8; V 0.05-0.5; One or more rare-earth metals of the Ce, La, Pr, Nd 0.0001-0.01; group Ti 0.1-2.0; One or more elements of the Nb, Ta, Zr, Hf 0.05-0.2; group Fe the rest 